In a broad sense, biomass includes all plants, microorganisms, and animals eating plants and microorganisms and their wastes; while in a narrow sense, biomass mainly refers to lignocellulose (called lignin for short), such as straws, woods and the like other than grains and fruits, in agroforestry production processes, leftovers of agricultural product processing industries, agricultural and forestry wastes, and poultry and livestock feces and wastes in animal husbandry production processes. Common representative biomass includes crops, agricultural wastes, wood, wood wastes, animal wastes and the like. Biomass has become an important part of renewable energy due to its characteristics of renewability, low pollution and wide distribution. Therefore, high-efficiency development and utilization of biomass energy will play a great positive role in solving energy and ecological environment problems.
At present, biomass energy is mainly developed and utilized in the following ways: (1) direct combustion: biomass is directly combusted, and heat produced by combustion is directly utilized or is utilized to further generate electric energy; (2) physical-chemical process: biomass is processed into briquettes through physical-chemical treatments and compression molding; (3) biochemical process: biomass is hydrolyzed and fermented to produce ethanol, or biomass is processed to produce biogas through biogas technologies; and (4) thermochemical process: biomass is gasified to produce biomass gas, or biomass is subjected to pyrolysis to produce charcoal or a biomass crude oil, or biomass is directly liquefied to produce a liquefied oil, and the like; wherein said process that biomass is directly liquefied to produce a liquefied oil is generally carried out as follows: a biomass and hydrogen gas directly react under the action of a catalyst under a high pressure, and the biomass is converted into a liquid fuel.
Chinese patent CN 102127462 A has disclosed a direct biomass hydroliquefaction process comprising two ebullated bed hydroconversion steps. The process comprises the following steps: a) a step of preparing a suspension of biomass particles in a solvent, preferably a hydrogen-donor solvent; b) a first hydroconversion step carried out in the presence of hydrogen gas by reacting the suspension in at least one reactor containing an ebullated bed catalyst and operating at a temperature of 300-440 DEG C. under a total pressure of 15-25 MPa with an hourly mass velocity of 0.1-5 h−1 and a hydrogen/feed ratio of 0.1-2 Nm3/kg; and c) a second hydroconversion step carried out in the presence of hydrogen gas by reacting at least a proportion of an effluent obtained in the step b) in at least one reactor containing an ebullated bed catalyst and operating at a temperature of 350-470 DEG C. under a total pressure of 15-25 MPa with an hourly mass velocity of 0.1-5 h−1 and a hydrogen/feed ratio of 0.1-2 Nm3/kg. At last, an effluent obtained at the end of the second hydroconversion step is subjected to separation for separating out a gas phase, an aqueous phase, at least one light fraction of liquid hydrocarbons of naphtha, kerosene and/or diesel type, a residue and the like. According to the above process, a biomass raw material is directly liquefied into liquid substances through the two second hydroconversion steps, and the residue is separated out at the bottom of the reactor at the end of the second hydroconversion step, however, such operation mode has the following defects: 1) the residue carries too much liquid substances, which causes waste of the liquid substances and influences yield of the liquid substances; and 2) a part of the residue suspends in the liquid-phase substances and enters the next working procedure, which causes high solid content of residue in the final light oil.